1. Field of the Invention
The present invention relates to a zoom lens applicable as an optical system to an imaging device such as digital still camera, digital video camera, silver halide camera as well as to an information device such as data terminal device incorporating such a zoom lens.
2. Description of the Related Art
In recent years a digital camera including a zoom lens as an optical system has been popular. Especially, a zoom lens with a focal length range including 50 mm in 35 mm equivalent is well known. There have been users' strong demands for development of a compact-size, wider angle, high-speed autofocus (AF) zoom lens.
Japanese Patent Application Publication No. 3-228008 (Reference 1), Japanese Patent No. 3716418 (Reference 2), No. 3397686 (Reference 3), No. 4401451 (Reference 4), and Japanese Patent Application Publication No. 2010-175954 (Reference 5) disclose positive lead type zoom lenses including a first lens group having a positive refractive power and a second lens group having a negative refractive power and the following lens groups, for example. This type can easily enlarge zoom ratio and can be very compact in the total length so that various products are available.
The zoom lenses disclosed in the above documents are of inner focus type. According to the one in Reference 1 a second lens group is moved to focus an object while according to the ones in Reference 2 to 5 a third lens group with a negative refractive power is moved to focus an object.
The zoom lens in Reference 1 has a drawback in that due to a large weight of the second lens group as a focus lens, a motor or actuator therefor needs to be large in size, increasing the maximal diameter of a lens barrel. Moreover, it is difficult to heighten AF operation speed and reduce noise during video shooting.
The focus lenses of the zoom lenses in References 2 to 4 are not light enough in weight. The third lens group of the zoom lens in Reference 5 as a focus lens is comprised of a single negative lens, therefore, light in weight. This can achieve high speed AF operation and the lens barrel thereof can be downsized. However, it needs to be improved in terms of zooming operation and downsizing.